(1). Field of the Invention
The present invention concerns a method for separation of solid plastic mixtures based on magneto-Archimedes levitation.
(2). Related Art Statement
There have been already several reports on the diamagnetic magnetic levitation (E. Beaugnon and R. Tournier, Nature, vol. 349, p 470, 1991, etc.). It is known that water droplets and plastic particles are magnetically levitated in the air. In addition, it is reported that if a pressurized paramagnetic gas (e.g. oxygen gas) is used for a suspending medium, the diamagnetic levitation is possible in a moderate magnetic field, and even paramagnetic materials may be levitated (Ikezoe et al., Nature, vol. 393, p 749, 1998). This phenomenon is called magneto-Archimedes levitation. In magneto-Archimedes levitation, the gravitational force and the magnetic force exerted directly on a suspended substance are balanced with the buoyancy and the magnetic force exerted by the paramagnetic suspending medium through the surface of the substance. Namely, (1) gravitational force, (2) buoyancy, (3) magnetic force acting on the suspended substance; and (4) magnetic buoyancy caused by the suspending medium.
Separation is necessary for recycling or dumping of the discarded plastics with small environmental loads. For this purpose, there have been so far proposed separation methods including a method in which spectrum patterns of plastics are analyzed at a high speed. Separation of powder in a high-pressure oxygen gas using the magneto-Archimedes effect has also been reported.
However, the conventional methods were insufficient to deal with a large amount of used plastics at a high speed and a low cost.
The inventors have found that various kinds of plastics can levitate at different locations in paramagnetic supporting liquid in balance with the gravitation and magnetic forces due to the difference in their magnetic susceptibilities and densities. In addition, this finding is easily applied to a continuous separation process.